A conventional battery pack disclosed in the Japanese Laid-open Patent No. H7-57721 is described with reference to drawings.
FIG. 10A is a cross sectional view, as viewed from the top, of a conventional battery pack with key part, where part of the case have been cut off. FIG. 10B is a cross sectional front view with key part, where part of the case have been cut off.
Referring to FIG. 10A and FIG. 10B, a battery pack 12 comprises a first battery cell 1, a second battery cell 4, and a thermistor 7 with leads, which have been packed in a case 9. The first battery cell 1 has a protruding positive terminal 2 and a negative terminal 3 provided at the opposite surface. The second battery cell 4 likewise has a protruding positive terminal 5 and a negative terminal 6. The PTC thermistor 7 with leads is connected, both mechanically and electrically, with the pair of leads to the positive terminal 2 of first battery cell 1 and the negative terminal 6 of second battery cell 4, respectively. The case 9 is provided with through holes 10, 11 for leading electrical contacts from the negative terminal 3 and the positive terminal 5 out of the case.
Now a PTC thermistor used in battery pack 12 is described referring to the drawings. FIG. 11A is a top view of a PTC thermistor with leads, FIG. 11B is a cross sectional view of the thermistor sectioned along the line A—A of FIG. 11A. Referring to FIG. 11A and FIG. 11B, a PTC thermistor 15 with leads comprises a polymer PTC layer 16, a pair of electrode layers 17 provided on both surfaces of the polymer PTC layer 16 with electrical contact thereto, a pair of lead terminals 18 attached to the electrode layer 17, and an insulating resin 19 covering the polymer PTC layer 16 and the electrode layers 17, and part of the lead terminals 18.
Operation of the above-configured PTC thermistor with leads and the conventional battery pack is described below. First, operation of the PTC thermistor with leads is described.
In FIG. 11B, the polymer PTC layer 16 is formed of a mixture of polyethylene or the like crystalline resin and carbon black or the like conductive particles. Resistance of the polymer layer shows a steep increase (or decrease) at the neighborhood of a temperature several degrees below melting point of the polymer material, because of a change in the gap between the conductive particles caused by thermal expansion (or shrinkage) of the polymer. Because of the above characteristic, the resistance of PTC thermistor 15 with leads, which is formed of the polymer PTC layer 16, electrode layers 17 and lead terminals 18, exhibits a remarkable increase in a temperature environment above the melting point of the polymer; it goes up to a far higher value (approximately 10000 times) than that in normal temperature. The resistance of thermistor 15 returns to the initial level when the temperature is lowered to normal temperature. Thus it can be used as a circuit protection component. Due to the hysteresis characteristic, the resistance returns to a level that is higher than the initial value (approximately 1.5 times that the initial value). The higher value does not bring about any material difficulties in normal practice. However, in a case where the resistance after restoration can not be neglected as the inner resistance of a circuit, it can be lowered to the initial level by leaving the thermistor in a temperature environment lower than melting point of the polymer by 30° C.–60° C., or leaving in an environment where the temperature moderately shifts between the above temperature and normal temperature. By so doing, the resistance can be restored to the initial level. Such process aimed to restore the resistance to the initial level is called annealing, and a phenomenon of reducing the plus-ward shift of resistance implemented by the thermal treatments is called annealing effect.
Thus, by taking advantage of the property that the resistance increases or decreases reversibly, it can control an overcurrent to a micro current; in such a way that an overcurrent causes a self heat generation, which raises the temperature to a level at which the resistance goes steeply up (hereinafter the temperature is referred to as protection-operation temperature). Once the power supply is cut off, temperature of the PTC thermistor goes low. By removing causes of the overcurrent, it can be used repeatedly. The terminology reversible property in the resistance of a PTC thermistor used here is defined to be including a property with which the resistance returns to the initial level by the annealing.
Now operation of a conventional battery pack is described. The circuit of battery pack 12 shown in FIG. 10A, FIG. 10B is formed of a battery cell 1 and a battery cell 4 connected in series by a PTC thermistor 7 with leads. The battery pack 12 is a battery unit whose negative terminal 3 and positive terminal 5 are connected to respective power supply terminals of an electronic apparatus through terminal holes 10 and 11. Suppose the battery cell 1 and the battery cell 4 are connected simply with a metal lead wire instead of the PTC thermistor 7 with leads, and there can be a possibility of short-circuit trouble arising in the power supply line of electronic apparatus. Or, even when the battery pack is not connected to an electronic apparatus, there can be a possibility that the positive terminal 5 and the negative terminal 3 are connected by chance with a metal or other high-conductive material. When such an inadvertent situation occurs, the battery cell 1 and the battery cell 4 generate heat by the overcurrent, then the inner pressure may destroy the battery cell itself. A PTC thermistor 7 with leads is a safety measure against such troubles. The operating principle of the safety measure is: the resistance of a PTC thermistor connected in series between the battery cells is raised sharply by the self-heat generation caused by an overcurrent, and the overcurrent is suppressed to a safe level low enough not destroying the battery (hereinafter referred to as overcurrent protection operation). When using a PTC thermistor as the overcurrent protection component, it needs to be selected properly taking into account the operating life of battery pack. At the same time, a thermistor should have a lowest possible resistance at the normal temperature, in view of less consumption in battery pack. A preferred value of the resistance is not higher than 40 mΩ, more preferably not higher than 20 mΩ.
The above-described configuration refers mainly to a basic structure of a battery pack used as primary cell. Recently, however, lithium ion battery and the like high performance secondary cells are being widely used in portable telephone units and the like apparatus. And, there are increasing needs for battery packs having a built-in protection circuit which controls charging/discharging. In the following, a conventional battery pack having a built-in protection circuit, and the method of manufacture are described referring to drawings.
FIG. 12 is a perspective view of a conventional battery pack with built-in protection circuit, which battery pack comprising a prismatic battery cell 25, a printed circuit board 28, a protection IC 29 mounted on a first mounting surface of the printed circuit board 28, and an FET (Field Effect Transistor) unit 30 consisting basically with two FETs. The prismatic battery cell 25 is provided with a positive terminal 26 which works also as the outer metal casing and a negative terminal 27 which is disposed on one surface alone among the surfaces of the prismatic battery cell 25. The printed circuit board 28 is provided with a positive lead out terminal 31 and a negative lead out terminal 32 on a second mounting surface that is opposite to the surface on which the protection IC 29 is mounted. A PTC thermistor 33 with leads is attached to the prismatic battery cell 25 so that part of it is in proximity to the two surfaces of battery cell. One lead 34 among the leads of the PTC thermistor 33 with leads is connected welded to the negative terminal 27, while the other lead 35 is connected with the printed circuit board 28. The positive lead out terminal 31 of battery pack and the positive terminal 26 which works also as the outer metal case are electrically connected by a connection lead 36.
Method for manufacturing the above-configured conventional battery pack with built-in protection circuit is described below. The printed circuit board 28 is printed on the second mounting surface with a cream solder by means of screen printing process. On the printed cream solder, the positive lead out terminal 31 and the negative lead out terminal 32 are attached and soldered thereon. And then, cream solder is applied on the first mounting surface of printed circuit board 28 by means of screen printing process. On the printed cream solder, the protection IC 29 and the FET unit 30 are attached, and these components are soldered on the printed circuit board 28 through a reflow soldering furnace. Meanwhile, the lead 34 of PTC thermistor 33 with leads is electric-welded on the negative terminal 27 of prismatic battery cell 25, and the connection lead 36 to the positive terminal 26. And then, the lead 35 of PTC thermistor 33 with leads which has been attached to the prismatic battery cell 25 is connected soldered on the soldered printed circuit board 28. Finally, the connection lead 36 is connected with solder on the printed circuit board 28 to provide a finished conventional battery pack having protection circuit.
Operation of the above-configured conventional battery pack having protection circuit is described referring to FIG, 13, a circuit block diagram which corresponds to the battery pack shown in FIG. 12.
The battery pack with protection circuit comprises a protection IC 29, an FET unit 30 formed of a first FET 37 and a second FET 38, and a PTC thermistor 33 with leads. This performs a control operation for protecting the prismatic battery cell 25 from the over charging and the over discharging, as well as a protection operation for protecting it from an overcurrent which might occur as a result of short-circuiting at load or at the protection circuit itself Typical operating principle is as follows: (a) Against over charging: As soon as voltage of the prismatic battery cell 25 reaches a certain specified value, gate of the second FET 38 is turned OFF (with the first FET 37 in ON state) to discontinue the charging current. (b) Against over discharging: When voltage of the prismatic battery cell 25 went low, gate of the first FET 37 is turned OFF (with the second FET 38 in ON state) to halt the discharge current. (c) Against short-circuiting at load: Shifts occurred in the ON resistance of FET unit 30 is detected to halt the short-circuit current. Or, the PTC thermistor 33 with leads, in its overcurrent protection operation, limits the current to a safe level. (d) Against short-circuiting within protection circuit: The PTC thermistor 33 with leads, in its overcurrent protection operation, limits the short-circuit current to a safe level.
As to the electrode-to-electrode contact, among other troubles, which could be brought about by an erroneous usage, a double- or triple-safety measure is to be provided. For a case in which the safety circuit failed to operate, a PTC thermistor 33 with leads is expected to play an important role of protection. Furthermore, by disposing a PTC thermistor 33 with leads in proximity to the prismatic battery cell 25, the resistance is raised through a direct heat conduction of a heat generated by the prismatic battery cell 25 itself. Thus it can move to a state of protection operation.
However, the above-described configuration requires during the manufacturing an assembling step of attaching a PTC thermistor 33 with leads to a battery cell and a printed circuit board. Furthermore, referring to FIGS. 11A and 11B, when a lead 18 is bent at a location close to the polymer PTC layer 16, a stress due to the bending work might cause a crack within the PTC thermistor 15 with leads, at some part between the polymer PTC layer 16 and the electrode layer 17. When the lead 18 is welded at a location close to the polymer PTC layer 16 with the electrode terminal of battery cell, the welding heat might cause a deterioration with the polymer PTC layer 16, resulting in an increased resistance of the PTC thermistor 15 with leads. Thus conventional battery packs have problems that there are fear of shortening operating time and deteriorating sensitivity in the operation.
These factors of resistance shift have a bad influence on the reversible property of a polymer PTC thermistor with respect to temperature, so these factors are to be considered as the deterioration in performance. If a lead 35 of PTC thermistor 33 with leads is soldered on a printed circuit board 28, as illustrated in FIG. 12, at a temperature higher than the melting point of polymer, the resistance increases to approximately 1.5 times plus-ward. If the heat treatment temperature is higher than 240° C., the resistance increases to approximately 2 to 3 times; so, the PTC thermistor can not keep the inner resistance (resistance at normal temperature) in a lower state. Thus the conventional battery pack has a problem that leads to a shorter operating time.